Portable working machines of the above defined type are known since long. They are often used for cutting concrete and similar materials. The cutting creates a lot of abrasive dust. Without air cleaning the engine will wear out in less than half an hour of operation. Efficient air cleaning therefore is vital and is attained mainly through a big filter volume, which will increase the service life of the machine. For example, a machine of this type, which also includes centrifugal cleaning of the air before the air enters the filters, is described in U.S. Pat. No. 5,438,965. This machine, its precursors and successors, belonging to the applicant's “Partner® K650-family”, and variants of it, have been and are still widely used in the construction industry for cutting concrete and the like, and also by fire brigades for cutting holes in roofings or for penetrating vehicle wrecks, and similar tasks.
However, the air cleaning system of the machine described in U.S. Pat. No. 5,438,965 also has some shortcomings. For example, it requires a plurality of sealing strings which need to be accurately accommodated in narrow grooves, where they shall be compressed in order to provide the necessary sealing efficiency. These and other circumstances makes it cumbersome to dismount and to reassemble the filter system in connection with change or maintenance of the filters.
Other problems of the machine described in U.S. Pat. No. 5,438,965 mainly have to do with a traditional thinking as far as the general layout of the machine unit is concerned. For example, the engine cylinder with its bore is conventionally inclined somewhat rearwards relative to a base line of the entire machine and/or to the bottom of the machine unit. This tends to limit the available space for other important components of the machine unit than the filters, such as the muffler. This in turn restricts the possibilities to give the said components an appropriate design and shape. Therefore it has been suggested to incline the engine cylinder with its bore forwards in a direction towards the tool unit, which makes it possible to increase the volume of the muffler, if the muffler is located in the front, bottom part of the machine unit. This however sets new limits as far as the filter system is concerned.